Localized corrosion monitoring has been a challenge to corrosion engineers and plant operators, because most of the presently available corrosion monitoring techniques are for general corrosion, but not sensitive enough for localized corrosion (see L. Yang and N. Sridhar, “Monitoring of Localized Corrosion,” in ASM Handbook, Volume 13A-Corrosion: Fundamentals, Testing, and Protection, Stephen D. Crammer and Bernard S. Covino, Jr., Eds., ASM International, Materials Park, Ohio, 2003, pp 519-524). Electrochemical coupled multielectrode array sensors (see U.S. Pat. No. 6,683,463 and No. 6,132,593) have been used for monitoring localized corrosion rates. With the coupled multielectrode array sensors, the localized corrosion rate was derived from the external anodic currents flowing into the anodic electrodes. In less-corrosive environments, the corrosion current may not always be equal to the external anodic current flowing into the anodic electrode in a multielectrode array sensor [see L. Yang, D. Dunn and G. Cragnolino, “An Improved Method for Real-time and Online Corrosion Monitoring Using Coupled Multielectrode Array Sensors,” CORROSION/2005, paper no. 05379, (Houston, Tex.: NACE International, 2005)]. It is highly desirable to have an independent method to verify the localized corrosion rates obtained from the coupled multielectrode array sensor or to independently measure the localized corrosion rate in these environments.
Two-electrode penetration devices were reported for corrosion monitoring 40 years ago (see U.S. Pat. No. 3,259,461). In the two-electrode penetration device, a metal specimen was machined into the shape of a liquid container with a known wall thickness. The container was filled with a non-corrosive or less-corrosive liquid (distilled water, for example). The conductivity of the liquid inside the container was measured. During the test, the container was immersed in a corrosive medium to measure the corrosion of the specimen in the medium. When the specimen was perforated by the corrosive medium, the conductivity of the liquid inside the specimen would increase, because the conductivity of the corrosive medium is usually much higher than that of the less-corrosive liquid. The corrosion rate was calculated by dividing the wall thickness of the specimen container by the time needed for the corrosive medium to perforate the specimen wall. Similar penetration concepts using single-layer and single-thickness of specimens were also used to study the pitting corrosion of aluminium foil [see F. Hunkeler and H. Bohni, “Determination of Pit Growth Rates on Aluminium Using A Metal Foil Technique,” Corrosion, Vol 37, pp. 645-650 (1981) and A. Sehgal, et al., “Pit Growth Study in Al Alloys by the Foil Penetration Technique,” J. Electrochem. Soc., Vol 147, pp. 140-148 (2000)].
These single-layer and single thickness penetration detection devices are excellent for studying corrosion in laboratories, because they not only measure general corrosion, but also measure localized corrosion, such as pitting corrosion. However, they cannot be used as real time sensors to track the changes of corrosivity or the progress of corrosion damages to a system component in the field, because they can only provide a single measurement of a corrosion rate. The present invention describes a method that incorporates a large number of similar specimens that have different wall thickness into an integrated one unit that provide a large number of measurements at different times. Therefore, this type of integrated unit is suitable for corrosion monitoring, especially for localized corrosion penetration rate measurements near real-time.
REFERENCE NUMBERS OF DRAWINGS10Common electrode20Multichannel impedance meter25Multichannel ammeter, zero-resistance ammeter or othercurrent-measuring unit26Multichannel voltmeter, or other voltage-measuring device30Multiple electrodes31First of multiple electrodes32Last of multiple electrodes40Multiple holes, each accommodating one of the multiple electrodes41First of multiple holes42Last of multiple holes50Metal of interest whose corrosion rate is to be measured60Corrosive medium (electrolyte)70Flat surface in parallel with multiple holes71Surface in parallel with multiple holes80Insulating spacer for electrically isolating electrodes 30 from thewall of the holes, but allowing the corrosive medium to contact theelectrodes so that the impedance measured by 20 is low or currentmeasured by 25 is high if the hole is filled by the corrosivemedium90Thickness of wall (d) between multiple holes 40 and flat surface 70(or 71). The d value for the first hole is smallest and the d value forthe last hole is largest. When corrosion penetrates the wall, thecorrosive medium would fill the hole.95Epoxy or other sealant96Protection tube or epoxy for inserting the probe into a processstream100Multiple metal tubes whose corrosion rate is to be measured; eachaccommodates one of the multiple electrodes 30101First of multiple tubes102Last of multiple tubes105Epoxy or other sealants or weldments106Epoxy or other sealants to seal the cavities formed by each layer ofthe spiral-wound foil or sheet.109Multiple metal tubes whose corrosion rate is to be measured. Thetubes are stacked together and the bottom end of each tube isclosed or sealed with epoxy or other sealants. Each tubeaccommodates one of the multiple electrodes 30.110Multiple metal tubes whose corrosion rate is to be measured. Thetubes are formed by spirally winding a piece of large foil into acylindrical form and sealed at one side and at the end with epoxyor other sealants 105 and 106.111The smallest of multiple tubes112The largest of multiple tubes115Multiple metal foil disks or sheets whose corrosion rate is to bemeasured.116The first layer of multiple sheets117The last of multiple sheets120Multiple wires whose corrosion rate is to be measured121First of multiple wires (thinnest)122Last of multiple wires (thickest)130Joints of multiple wires to the common electrode150Single impedance meter160Multiple impedance boxes, each having a unique value orcharacteristics161First of multiple impedance boxes with a unique value162Last of multiple impedance boxes with a unique value